Q

The keyword command :q typed at the top-level of the ACL2 loop will
terminate the loop and return control to the Common Lisp top-level
(or, more precisely, to whatever program invoked lp). To reenter
the ACL2 loop, execute (acl2::lp) in Common Lisp. You will be in
the same state as you were when you exited with :q, unless during
your stay in Common Lisp you messed the data structures
representating the ACL2 state (including files, property lists,
and single-threaded objects).

Unlike all other keyword commands, typing :q is not equivalent to
invoking the function q. There is no function q.